This invention relates to a stator position feedback controller in a system in which a stator is supported for motion with respect to a machine base.
Co-pending U.S. patent application Ser. No. 10/155,058 filed May 24, 2002 entitled “Reaction Force Transfer System” is directed to a precision machine for controlling the position of a work piece relative to a tool. As taught in that patent application, the stator of a linear motor, rather than being rigidly affixed to a machine base, is allowed to move with respect to the machine base. The stator itself is connected to an external frame in a manner to constrain the stator to a single degree of freedom of motion. The decoupling of the stator from being rigidly affixed to the machine base eliminates excessive motion of the machine base that would otherwise be generated by the reaction force as the carriage is accelerated thereby minimizing detrimental side effects of such motion.
In some prior art actuators the electrical control of the actuator requires accurate feedback of the position of the actuator's moving element with respect to the actuator's stator. When, as in the case of an actuator's stator being rigidly connected to a machine base, the feedback source of the moving carriage used for position control can also be used to develop the electrical signal to the actuator. This signal is commonly known as “commutation” based on the manipulation of current through motor phases. The position of the actuator's moving element relative to the actuator's stator is correlated to the electrical controls through some initialization process and from that point on the relative position is continuously updated according to the moving carriage's position feedback source information.
Because the stator in the patent application noted above is not rigidly connected to the machine base it is free to move relative to the machine base in a direction of thrust. Thus, the relative position between the actuator's moving element and the actuator's stator is not accurately represented by the moving carriage's position feedback source. In some actuators such as linear motors, performance may be degraded because at any given motor current the developed thrust is reduced according to the deviation from exact “commutation” position. This potential problem was discussed in the above noted patent application.
One possible solution to this problem is to install an additional position feedback device such as an encoder with the encoder readhead attached to the actuator's moving element (or to the moving carriage) and its scale affixed to the actuator's stator. This feedback device would then be used for the purpose of “commutation” alone. Because the encoder scale is rigidly coupled to the actuator's stator, the signal from this feedback device always represents the true relative position between the actuator's moving element (carriage) and the actuator's stator. This solution, however, is expensive because the axis travel may be long requiring a long encoder scale; in addition, another encoder readhead will have to be mounted to the moving carriage requiring another signal cable and increasing cable management complexity.